A “Toolbox” for Forensic Engineers

A “Toolbox” for Forensic Engineers 95
been manufactured and heat treated. It is particularly useful for seeking
internal defects in welded or brazed assemblies. 2,3
In other words, the microstructure of a material can be equated to a
fingerprint taken during the course of a criminal investigation. To illustrate
the unique nature of material microstructures, the following section will
present a review of the chemical makeup, properties and associated microstructures
of cast irons. Chapter 5, Section 5.4 demonstrates the importance
of distinguishing between different forms of cast iron that is wholly dependent
on observing the microstructure of a failed casting.
4.4.5.1 Cast Irons
Carbon not only reduces the melting point of iron from 1530°C for pure
iron to 1143°C when 4.3% carbon is present, but this element also significantly
increases the fluidity which enables very good castings to be made.
Moreover, when the liquid metal solidifies in the mold and the carbon separates
as graphite, there is practically no volume contraction as occurs with
other metals. The castings are free of shrinkage effects that are sometimes
very troublesome when castings are made in other metals such as steel, bronze
or aluminum. The family of iron-based alloys that contain between 2.5 and
4.5% carbon are therefore extremely useful engineering materials and are
called cast irons. As their name implies they are used solely to make castings,
as they cannot be hot or cold worked like steels that contain less than 1%
carbon.
There are several varieties of cast iron, their properties depending very
much on the form (the “morphology”) in which the carbon separates during
the solidification process. This is determined by their composition and,
equally important, the rate at which they cool in the mold, and sometimes,
with certain types, by any heat treatments applied to the solidified castings.
Traditionally cast irons were regarded as always brittle, but over the last half
century methods have been introduced for modifying the form of the graphite
during solidification or by subsequent heat treatment of castings in which
the carbon is in the form of iron carbide (Fe 3C). The result is many varieties
of cast irons that exhibit respectable degrees of ductility and toughness.
The traditional “gray” cast irons, so called because they expose a dull
gray fracture when broken, are brittle and display no ductility whatsoever.
They are nevertheless quite strong and easy to cast, and are widely used for
general engineering castings that do not need to possess ductility, for example,
the bed for a lathe or a press. The reason for their inherent brittleness
is that the graphite present in their microstructures is in the form of flakes.
These flakes take a three-dimensional form similar to the leaves of lettuces
packed in a box, where the lettuce leaves are the graphite and the surrounding
“air” ranging from the equivalent of pure iron to a high-strength steel. In a